The invention relates to a process for distributing a pulp suspension, particularly at medium consistency, upstream of a dewatering device. In addition, it relates to a device for implementing the process.
As a rule, pulp suspensions are dewatered on Fourdrinier machines, i.e. machines where the suspension is dewatered on a bottom wire, and on twin wire presses, where the suspension is fed in and dewatered between two wires.
For xe2x80x9cthickerxe2x80x9d suspensions, e.g. in the medium-consistency range, twin wire presses with an open headbox are used in most cases. In order to guarantee adequate distribution of the pulp suspension over the belt width of the twin wire press, special measures are required, such as additional distribution rolls, suitable slide panels, or similar. At high consistencies, however, this does not yet guarantee complete and even distribution, resulting in different thicknesses of material web being produced over the machine width. Particularly in subsequent washing processes and for even dewatering, however, this can be a considerable disadvantage. Even with the means otherwise used for homogenizing, it is impossible to achieve subsequent even distribution over the width of the web or the machine.
Patent WO-A-8805840 (Ahlstrom) describes a process and a device for spreading chemical pulp over a belt. To improve distribution, the xe2x80x9chigh-consistencyxe2x80x9d suspension is fluidized by an agitator. This is achieved with high energy input and an agitator circumferential speed of at least 25 m/s, causing the suspension to be fluidized in the web formation point.
Patent U.S. Pat. No. 3,119,734 (Jordan) describes feeding of a chemical pulp suspension onto the wire belt of a paper machine. A perforated roll 30 with toothing made of rubber is provided here to distribute the (low-consistency) suspension.
The aim of the invention is to avoid the above mentioned disadvantages, and to guarantee good feed and even distribution of the suspension over the dewatering device for all consistencies of the pulp suspension, particularly in the medium-consistency range. for the suspension to flow evenly over the width of the machine to a large extent and be distributed.
An advantageous further development of the invention is characterized by fluidization taking place in an enclosed area, which means that a constant pressure can be set independently of the flow rate, thus achieving constant, even distribution.
A favorable configuration of the invention is characterized by the pulp suspension having a consistency between 0.5% and 18%, preferably between 5% and 12%. Optimum distribution is obtained particularly in this consistency range due to fluidization of the pulp suspension.
A favorable further development of the invention is characterized by fluidization taking place at a pressure above atmospheric. At higher pressure it is possible to achieve a higher throughput and dewatering capacity on the one hand, and on the other, improved homogenizing of the suspension is achieved over the web width.
An advantageous configuration of the invention is characterized by the suspension being fed to the fluidization process after already having been predistributed. In this case, even better distribution of the suspension is achieved at the inlet to the dewatering machine.
A favorable further development of the invention is characterized by the cross-profile being controlled after fluidization. This allows compensation of the various influencing factors that are difficult to control.
An advantageous further development of the invention is characterized by the speed of the rotor being controlled as a function of the differential pressure between inlet and outlet. Thus, the optimum output can be set for the rotor.
In addition, the invention refers to a device to implement the process for distribution of a pulp suspension, particularly at medium consistency, upstream of a dewatering device. It is characterized by an enclosed device with a fluidizing rotor being installed upstream of the dewatering device. Due to this rotor the pulp suspension can be reduced effectively to a liquid state, particularly at medium consistency, which means it can be distributed easily over the web width.
A favorable further development of the invention is characterized by the fluidizing rotor having burls, strips, or profiles, such as polygons, spikes or comb-shaped built-in components. Fluidization of the pulp suspension can thus be achieved in a favorable way.
An advantageous configuration of the invention is characterized by the gap between the rotor and the walls of the casing being essentially constant all the way round. This means that the suspension can flow evenly round the rotor in both directions, which causes even better swirling and thus, better fluidizing of the suspension.
An advantageous further development of the invention is characterized by the casing having burls, strips, or profiles, such as polygons, spikes or comb-shaped built-in components, with a gap of between 1 and 100 mm between the rotor and the casing or between whatever built-in components are used. This is a favorable means of preventing the suspension from turning together with the rotor. Thus, the efficiency of the fluidization process is enhanced.
A favorable configuration of the invention is characterized by the casing having a circular cross-section, however it can also have a triangular or polygonal cross-section. The angular shape, in particular, assists fluidization of the suspension in an advantageous manner, whereas the circular shape is simpler and cheaper to manufacture.
A favorable further development of the invention is characterized by a cross-flow distributor, funnel-type headbox, or several feed pipes being provided upstream of the distribution device. If the suspension is pre-distributed, it can be fluidized more easily and then distributed over the width of the machine more evenly.
A favorable configuration of the invention is characterized by the outlet opening being located above a bottom wire. Thus, the suspension can be distributed evenly on a Fourdrinier wire in a favorable manner.
An advantageous further development of the invention is characterized by the outlet opening being located between two wires.
An advantageous configuration of the invention is characterized by the outlet gap to the cross-profile control being pre-set, however it can also be adjustable. If the outlet gap is pre-set, any desired cross profile can be obtained in a favorable way. If the outlet gap is adjustable, it can be used to compensate any influencing factors that are difficult to control or which change from time to time.